The present invention relates to an improvement in the manufacture of alkali metal chlorates by electrolysis, and more particularly to a means of reducing losses in power efficiency due to the adverse effects caused by the presence of transition metals such as copper, nickel, iron and manganese by adding effective amounts of silicates, fluorides, polybasic hydroxyalkanoic acids and their salts, and sulfides.
Alkali metal (M) chlorates are produced by the electrolysis of aqueous alkali metal chlorides in accordance with the overall chemical reaction: EQU MCl+3H.sub.2 O.fwdarw.MClO.sub.3 +3H.sub.2
which consumes 6 faradays to produce a 1 gram-mole of alkali metal chlorate. The primary electrochemical reactions which occur during chlorate formation are assumed to be oxidation of chloride at the anode and reduction of water at the cathode as follows: EQU 2Cl.sup.- .fwdarw.Cl.sub.2 +2e EQU 2H.sub.2 O+2e.fwdarw.H.sub.2 +2OH.sup.-
It is thought that chlorine generated at the anode is hydrolyzed to form hypochlorite which then reacts further to produce the chlorate as follows. EQU Cl.sub.2 +H.sub.2 O.revreaction.H.sup.+ +Cl.sup.- +HOCl EQU HOCl.revreaction.H.sup.+ +OCl.sup.- EQU 2HOCl+OCl.sup.- .revreaction.2H.sup.+ +2Cl.sup.- +ClO.sub.3.sup.-
The cell power efficiency during electrolytic manufacture of chlorates is adversely effected by a variety of factors including a number of parasitic reactions which occur concurrently with those which result in chlorate formation. Many of these parasitic reactions are characterized by the evolution of oxygen. Therefore, the concentration of oxygen in the cell effluent gas is generally considered to be one measure of power inefficiency. One parasitic reaction resulting in oxygen evolution is the decomposition of the intermediate hypochlorite in the bulk of the electrolyte as follows. EQU 2HOCl.fwdarw.2H.sup.+ +2Cl.sup.- +O.sub.2
The rate of hypochlorite decomposition is greatly accelerated by transition metal cations, oxides, and/or hydroxides if they are present even at very low concentrations in the electrolyte. It is believed that the catalysis of hypochlorite decomposition by transition metal impurities contributes significantly to the production of oxygen and subsequent loss of power efficiency during electrolytic chlorate production.
A variety of innovations have appeared over the years directed at increasing the power efficiency in electrolytic chlorate manufacture. Salts containing oxyanions of hexavalent chromium have been added to the electrolyte and are used in conventional technology to inhibit the corrosion of steel cathodes and the cathodic reduction of hypochlorite and chlorate. A combination of sodium dichromate and molybdic acid have been added to the electrolyte during chlorate manufacture to achieve the same results using a greatly reduced concentration of hexavalent chromium, which causes problems in product purification and waste water treatment. Phosphorus-containing complexing agents have been added to the electrolyte to complex alkaline earth metal cations to reduce the buildup of scale deposits on metal cathodes permitting longer periods of uninterrupted satisfactory cell operation.